Increase cycles speed for interior render
It sounds like you are using a sun lamp for it, that really slows it down. So the only thing I can add to Gwenn answer is: for interior renders, use planes with an emission shader in every window hole, and disable the "camera" option in the "ray visibility" panel in the object properties so you don't see it when rendering. You can combine that with a sun lamp if you want hard edge shadows coming from the sun.

I use this method a lot and it really speeds things up.